1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and more particularly to a substrate having storage capacitors with a high aperture ratio.
2. Discussion of the Related Art
In general, since flat panel display devices are of thin design, low weight, and have low power consumption, they are increasingly being used for displays of portable devices. Among the various type of flat panel display devices, liquid crystal display (LCD) devices are widely used for laptop computers and desktop monitors because of their superiority in resolution, color image display, and display quality.
LCD devices have upper and lower substrates with electrodes that are spaced apart and face each other, and a liquid crystal material is interposed therebetween. Accordingly, when a voltage is applied to the liquid crystal material by the electrodes of each substrate, an alignment direction of the liquid crystal molecules is changed in accordance with the applied voltage to display images. By controlling the applied voltage, the LCD device provides various transmittances for rays of light to display image data.
Driving methods for driving the LCD device may be classified into one of a passive matrix driving method and an active matrix driving method. The passive matrix driving method uses a voltage difference induced between a data line (a video line) and a gate line (a scanning line), whereas the active matrix driving method uses a switching element, usually a thin film transistor. Currently, an active matrix LCD (AM LCD) device using the active matrix driving method is becoming increasing popular because of its high resolution and superiority in displaying video data. A typical AM LCD device has a plurality of switching elements and pixel electrodes that are arranged in a matrix array upon the lower substrate. Therefore, the lower substrate of the LCD device is commonly referred to as an array substrate. A common electrode that is made from a transparent conductive material is usually formed of the upper substrate of the LCD device. The lower substrate and the upper substrate are attached to each other using a sealant, and the liquid crystal material can be interposed between the upper and lower substrates.
The pixel electrode formed on the lower substrate and the common electrode formed upon the upper substrate form a liquid crystal capacitor, and a data signal and a common signal are applied to the pixel electrode and the common electrode, respectively. Then, a voltage difference is induced between the pixel and common electrodes to electrically charge the liquid crystal capacitor. However, although the voltage applied to the liquid crystal capacitor should be sustained until a next data signal is applied, electrical discharges generally occur at the liquid crystal capacitor. Accordingly, to prevent the electrical discharges and maintain the charge upon the liquid crystal capacitor, a storage capacitor is usually interconnected to the liquid crystal capacitor. In addition, the storage capacitor serves to stabilize gray level displays, prevent flicker, and prevent retention of residual images.
There are at least two possible configurations for the above-mentioned storage capacitor. The first configuration includes a capacitor electrode that is additionally formed on the lower substrate, and a capacitor electrode that is connected to the common electrode to function as an electrode of the storage capacitor. The second configuration includes a portion of the gate line to be used as an electrode of the storage capacitor. For example, a portion of an (n-1)th gate line is used as the electrode of the storage capacitor for an adjacent (n)th pixel. The first configuration is referred as a storage-on-common (SOC) structure or an independent storage capacitor type. The second configuration is referred as a storage-on-gate (SOG) structure or a previous gate type.
FIG. 1 is a partial plan view of an array substrate according to the related art. In FIG. 1, gate lines 11 are arranged along a first direction and data lines 12 are arranged along a second direction perpendicular to the first direction of the gate lines 11. A pair of gate and data lines 11 and 12 define a pixel region P1, and a pixel electrode 20 is positioned within the pixel region P1. A thin film transistor (TFT) T1 is positioned at one corner of the pixel region P1 near the crossing of the gate line 11 and data line 12. The TFT T1 includes a gate electrode 13 that extends from the gate line 11, a source electrode 14 that extends from the data line 12, a drain electrode 15 that is spaced apart from and positioned opposite to the source electrode 14, and an active layer positioned beneath the source and drain electrodes 14 and 15.
In a middle portion of the pixel region P1, a common line 17 is disposed and a capacitor electrode 18 is formed over the common line 17. Accordingly, the common line 17 and the capacitor electrode 18 constitute a storage capacitor with an interposed dielectric layer (not shown). Although not specifically shown in FIG. 1, insulators cover and protect the gate lines 11, data lines 12, TFT T1, common line 17 and capacitor electrode 18. The insulators have first and second contact holes 19a and 19b formed over the drain electrode 15 and capacitor electrode 18, respectively. Thus, the pixel electrode 20 formed within the pixel region P1 contacts the drain electrode 13 and the capacitor electrode 18 through the first contact hole 19a and through the second contact hole 19b, respectively, thereby the pixel electrode 20 overlaps portions of the gate and data lines 11 and 12.
In FIG. 1, since the SOC storage capacitor is formed within the pixel region P1 using the common line 17, a voltage is prevented from leaking from the charged liquid crystal capacitor. However, the common line 17 and the capacitor electrode 18, which form the SOG storage capacitor are usually formed of opaque metallic material. Accordingly, light passing through the pixel region is partially interrupted by the common line 17 and capacitor electrode 18. In addition, the LCD device commonly includes black matrix formed in the upper substrate in order to prevent light leakage, wherein a margin of the black matrix is provided for compensating for any misalignment of the upper and lower substrates.
FIG. 2 is a partial plan view of display areas of the array substrate of FIG. 1 according to the related art. In FIG. 2, light only penetrates areas “A,” whereby only the areas “A” display image data. Accordingly, since an aperture ratio is significantly decreased because of a presence of the common line 17 and the capacitor electrode 18 disposed between the areas A, image quality of the image data is degraded.